In order to improve thermal efficiency of gasoline internal combustion engines, lean burn is known to give enhanced thermal efficiency by reducing pumping losses and increasing ratio of specific heats. Generally speaking, lean burn is known to give low fuel consumption and low NOx emissions. There is however a limit at which an engine can be operated with a lean air/fuel mixture because of misfire and combustion instability as a result of a slow burn. Known methods to extend the lean limit include improving ignitability of the mixture by enhancing the fuel preparation, for example using atomised fuel or vaporised fuel, and increasing the flame speed by introducing charge motion and turbulence in the air/fuel mixture. Finally, combustion by auto-ignition, or homogeneous charge compression ignition, has been proposed for operating an engine with very lean or diluted air/fuel mixtures.
When certain conditions are met within a homogeneous or close to homogenous charge of lean air/fuel mixture during low load operation, homogeneous charge compression ignition can occur wherein bulk combustion takes place initiated simultaneously from many ignition sites within the charge, resulting in very stable power output, very clean combustion and high fuel conversion efficiency. NOx emissions produced in controlled homogeneous charge compression ignition combustion are extremely low in comparison with spark ignition combustion based on propagating flame front and heterogeneous charge compression ignition combustion based on an attached diffusion flame. In the latter two cases represented by a spark ignition engine and a diesel engine, respectively, the burnt gas temperature is highly heterogeneous within the charge with very high local temperature values creating high NOx emissions. By contrast, in controlled homogeneous charge compression ignition combustion where the combustion is uniformly distributed throughout the charge from many ignition sites, the burnt gas temperature is substantially homogeneous with much lower local temperature values resulting in very low NOx emission.
Engines operating under controlled homogeneous charge compression ignition combustion have already been successfully demonstrated in two-stroke gasoline engines using a conventional compression ratio. It is believed that the high proportion of burnt gases remaining from the previous cycle, i.e., the residual content, within the two-stroke engine combustion chamber is responsible for providing the hot charge temperature and active fuel radicals necessary to promote homogeneous charge compression ignition in a very lean air/fuel mixture. In four-stroke engines, because the residual content is low, homogeneous charge compression ignition is more difficult to achieve, but can be induced by heating the intake air to a high temperature or by significantly increasing the compression ratio. This effect can also be achieved by retaining a part of the hot exhaust gas, or residuals, by controlling the timing of the intake and exhaust valves.
Homogeneous charge compression ignition combustion of a gasoline like fuel (or petrol like, or fuel with high octane number) requires a temperature of approximately 1100 K to achieve auto ignition. While it may be possible to operate in the HCCI mode over a significantly wide operating range of engine speeds and load, however only a part load portion of the engine's operational range in conventional spark ignited combustion mode can normally be used for HCCI operation. Using the above solution, relying on exhaust gas from the previous combustion, e.g. through providing a so called negative valve overlap (NVO) where the exhaust valve is arranged to be closed before top dead center during an exhaust stroke of the piston, for retaining exhaust gas from the previous combustion, is known to improve the operating range in HCCI mode.
Furthermore, U.S. Pat. No. 5,050,378 discloses how, in a spark ignition or compression ignition four cycle internal combustion engine, an exhaust expansion chamber is sized to produce a reflected exhaust pressure wave timed to an auxiliary reopening of the exhaust valve after the intake valve has effectively closed. The reflected exhaust pressure wave causes the re-entry into the cylinder of a quantity of intake charge (which has previously been drawn into the expansion chamber) subsequent to the effective filling of the cylinder through the intake valve, the result being a boost in cylinder charge and pressure on the compression stroke of the piston. At engine design speed, the power output of the engine is substantially improved over the power output without the boost in cylinder charge and in compression stroke pressure.
Although the power output of the engine is improved at engine design speed using the solution according to U.S. Pat. No. 5,050,378 from fuel economy and exhaust emission aspects, it is undesirable to allow intake charge to be drawn into the exhaust system.